Late to the party, but just cracked open my Elvis head.
The ICESDA, ICESCK, and ICE lines on the J22 connector on the main board are for an I2C (3 wire) interface to the Analog to Digitial converter (Sonix SNAD01A) Which is an 8 channel 10-BIT ADC chip. All of the motor pots are connected to this. There is one in the head and one on the main board.
Any custom controller will need to be able to either communicate with this or replace it with something else.